1. Field of the Invention
The present invention relates to an electrolyte for non-aqueous electrolyte electrochemical apparatus and a non-aqueous electrolyte electrochemical apparatus using the electrolyte. More particularly, it relates to a non-aqueous electrolyte secondary battery.
2. Description of Related Art
Non-aqueous electrolyte electrochemical apparatuses using light metals such as lithium and sodium as negative electrode active materials are used in a wide variety of the fields such as of various electric and electronic equipment. Non-aqueous electrolyte electrochemical apparatuses include batteries, capacitors for electric double layers, and the like. Especially, non-aqueous electrolyte secondary batteries are being intensively investigated and developed at present because they are rechargeable batteries which have high energy density and can be miniaturized and weight-saved.
Solvents for electrolytes used in the non-aqueous electrolyte secondary batteries include, for example, cyclic carbonate esters such as propylene carbonate and ethylene carbonate, chain carbonate esters such as diethyl carbonate and dimethyl carbonate, xcex3-butyrolactone, xcex3-valerolactone, and the like.
As materials for negative electrodes, carbon materials such as graphite are used. Negative electrodes evolve heat or gas due to the reaction of an alkali metal ion contained in the materials with an electrolyte at a high temperature. Furthermore, since the non-aqueous electrolyte secondary batteries show high voltage and high energy density, oxidative decomposition of solvent or solute may occur also on positive electrodes. These phenomena are conspicuous with increase of temperature, and reductive decomposition occurs on the negative electrode side and oxidative decomposition occurs on the positive electrode side during storage at high temperatures such as 60xc2x0 C. and 85xc2x0 C. to evolve a large amount of gas. Recently, these non-aqueous electrolyte secondary batteries are widely used also as electric sources for backup of notebook type personal computers. The notebook type personal computers always have an internal temperature of 45xc2x0 C. to 60xc2x0 C., and a constant voltage of 4.2 V is always applied for keeping a high capacity at the above high temperatures, which readily causes evolution of gas.
If a gas is evolved at high temperatures of batteries, the safety device is driven owing to rising of the internal pressure of the batteries, resulting in cut-off of current or deterioration of battery characteristics, and, thus, improvement has been strongly demanded.
In an attempt to solve the above problems, it has been proposed to use electrolytes containing additives which produce films on positive electrodes or negative electrodes. However, though these additives have the effect to inhibit evolution of gas, many of them form films of high resistance on the electrodes, and hence there are problems of causing deterioration of charging and discharging characteristics of batteries, particularly, characteristics of high rate discharging or low-temperature discharging.
Furthermore, when a mixed solvent of a cyclic carbonate ester such as ethylene carbonate and a chain carbonate ester such as dimethyl carbonate or methylethyl carbonate is used as a non-aqueous electrolyte, an ester interchange reaction due to the chain carbonate ester takes place on the electrodes, and alkoxide radicals such as methoxy group and ethoxy group are produced as intermediates.
Occurrence of the ester interchange reaction of unsymmetrical chain carbonate esters such as ethylmethyl carbonate can be readily confirmed by analysis while symmetrical chain carbonate esters such as dimethyl carbonate shows no changes in structure and hence occurrence of the ester interchange reaction can hardly be confirmed even by analysis. However, it is considered that ester interchange reaction also takes place in these symmetrical chain carbonate esters. The radicals produced by the ester interchange reaction are strong nucleophilic reagents and accelerate ring-opening and decomposition of ethylene carbonate to cause evolution of gas. Alternatively, they dissolve metals in the positive electrode active materials and break the crystal structure to deteriorate the characteristics. The dissolution of positive electrode active material is a serious problem in spinel type lithium manganate.
Moreover, ethylene carbonate which is widely used at present has a high melting point and, hence, has a problem that charge and discharge characteristics at low temperatures are markedly deteriorated when batteries are operated at low temperatures.
The present invention relates to a non-aqueous electrolyte electrochemical apparatus having at least two electrodes, a non-aqueous electrolyte prepared by dissolving a solute in a non-aqueous solvent and a separator interposed between the two electrodes, wherein the non-aqueous electrolyte contains at least a salicyl derivative represented by the formula (1) or a salicylidene derivative represented by the formula (2). 
wherein R represents CnH2nOH (n is an integer of 0-4) or NR1R2 (R1 and R2 are independently hydrogen or a substituted or unsubstituted alkyl group). 
(wherein R represents oxygen, Nxe2x80x94X (X is hydrogen, a hydroxyl group or a substituted or unsubstituted phenyl group) or CHCORxe2x80x2 (Rxe2x80x2 is a substituted or unsubstituted alkyl group)).
The salicyl derivative and the salicylidene derivative form a stable and low resistant organic film on positive electrodes and thus the non-aqueous electrolyte and the electrode does not directly contact with each other, and, as a result, gas evolution at the time of charging and storage at high temperatures and deterioration of the characteristics caused by dissolution of metals in the active materials, especially, materials of positive electrodes are inhibited. By using such a non-aqueous electrolyte, there is provided a novel non-aqueous electrolyte secondary battery which can be used in a wide temperature range, has a high energy density, is less in reduction of discharge capacity due to the repeated use of the battery, and is excellent in high rate charge and discharge characteristics.